Well, I'll toss in my two cents.  Here in the Pacific Northwest
ventilation is really important;  I found my incidence of chalkbrood
went way down when I got serious about winter ventilation.
 
I've got inner covers on all my hives, with screen over the openings
to prevent robbing and encourage use of the "proper" entrance.  I
have a variety of "outer" covers, all of which seem to work fine,
but someday I hope to get around to building some nice, thick
insulated tops to help keep the cluster warm in winter.  My hives
get morning sun and afternoon shade, so summer heat isn't much of
a problem.
 
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